1. Field of the Invention
This invention relates to a hydraulic piston and cylinder assembly for raising and lowering an elevator or platform. The rate of upward movement of a plunger, which can be selectively moved upwardly or downwardly within a cylinder and which supports the elevator within the elevator shaft, is slowed as the plunger reaches its maximum displacement from the cylinder.
2. Description of the Prior Art
In the operation of a hydraulic elevator, the elevator is typically supported in its shaft by a plunger received in cylinder. The plunger is moved upwardly and downwardly within the cylinder by hydraulic fluid, which is pumped into and out of the cylinder through a fluid conduit. As fluid is pumped into the cylinder, the plunger is displaced and thereby raised. The plunger is lowered by either pumping the fluid out of the cylinder or by opening a relief valve in the fluid conduit. When the plunger is raised to a position approximating its maximum height, a speed reduction device is usually employed to decelerate the rate of upward movement of the plunger as it approaches its full extension. This prevents the elevator from stopping abruptly when the plunger is displaced to its maximum extent from the cylinder.
U.S. Pat. No. 2,981,236 discloses a safety device for hydraulic elevators using a hydraulic cylinder and ram, which prevents over-travel of the ram at the upper end of the cylinder. This reference discloses a mechanism for exerting hydraulic back-pressure on the ram as it approaches its limit of upward travel, even though the hydraulic mechanism is continuing to supply liquid under pressure to the cylinder. The back-pressure is created by utilizing a tapered safety ring in the upper end of the cylinder and an outwardly extending cushion ring on the lower end of the ram. As the ram approaches the maximum extent of travel, the cushion ring moves into the safety ring thereby creating back-pressure. A slow-down of the ram is thus accomplished using different elements and method than that disclosed in the present invention.
U.S. Pat. No. 1,011,338 discloses a bottom automatic stop limit for plunger-type elevators. In this reference, a plunger travels through a cylinder which has a tapered sleeve disposed in the cylinder's bottom portion. The tapered bottom portion of the cylinder slows the traveling speed of the plunger as liquid is forced out more slowly from the narrowed bottom. In other embodiments, the bottom portion of the plunger is perforated to further regulate its speed.
U.S. Pat. No. 2,719,510 discloses a cushion construction for air cylinders. A cushion of air arrests the piston as it reaches the end of its stroke, by means of sealing plugs attached to each end of the piston. When the piston reaches the end of its stroke, the sealing plug blocks the port and the release of air, thus the piston is arrested by the air trapped between the end of the cylinder and the piston. This device is only applicable to air cylinders, because of its dependence upon the greater elastic properties of gases.
U.S. Pat. No. 2,710,595 discloses a fluid operated cylinder (hydraulic and pneumatic) in which either one or both ends of the cylinder are provided with adjustable cushions.
None of the known prior art contains all of the features and meets all of the objectives of the present invention. That is, to provide a support plunger or piston and cylinder assembly in which the flow rate of fluid into the cylinder is selectively and progressively decreased by the action of the plunger, as the plunger is raised. Further, the references of the prior art do not disclose such a speed reduction assembly which includes means to center the plunger concentrically within the cylinder and to prevent scarring of the plunger upon removal.